This invention is in the field of gas chromatography and particularly relates to flame detectors used in gas chromatography systems for analyzing the sample eluting from the chromatography column after separation by the column into its constituent components.
One type of flame detector, the flame ionization detector, is a well known and commonly used device for analyzing the output of a gas chromatograph. Examples of flame ionization detectors are found in U.S. Pat. Nos. 3,585,003 and 4,182,740 which are coassigned with the present invention. Flame ionization detectors are particularly useful for analyzing organic compounds.
In a flame ionization detector, sample exiting a gas chromatography column, where it is separated into its component compounds, is combusted in an air/hydrogen flame. Typically, the exit end of a gas chromatography column and a source of hydrogen fuel are located within a hollow flame jet and exit through an orifice where the mixture encounters an air flow which provides oxygen to support a flame. Ions created by the breakdown of the sample in the flame are transported by gas flow past an electrode, normally referred to as the collecting electrode. The collecting electrode is connected to a current measuring device where changes in ion current, corresponding to changes in the sample mixture, are read. In many commercial devices another electrode within the detector is polarized with a DC voltage of several tens to several hundreds of volts. Typically, a metal flame jet is used as this "polarizing electrode".
One problem associated with using metal flame jets is that the metal surfaces are not very inert at high temperatures and can interfere with the performance of the detector by adsorbing or otherwise affecting the sample eluting from the end of the chromatography column. This problem may be mitigated by locating the end of the column near the orifice of the flame jet, thereby minimizing the amount of metal surface likely to be contacted by the sample. However, this approach does not fully eliminate sample contact with the internal metal surfaces of the flame jet.
A trend in gas chromatography has been towards higher temperature operation. It is now possible to obtain a commercial gas chromatography system, the Varian Model 3410, which operates at temperatures as high as 500.degree. C. This trend has led to the greater use of aluminum clad chromatography columns instead of the polyimide clad columns previously employed, since polyimide cladding is unable to survive temperatures higher than approximately 350.degree. C. Using aluminum, which is a good electrical conductor, as the cladding material, however, dictates that the column end not be placed in close proximity to the orifice of a metal flame jet because of the possibility of electrical shorting.
Another solution to the problem encountered when the sample contacts metal surfaces is the use of a flame jet constructed of quartz, with either a metallic coating on the outer surface of the flame jet to form the polarizing electrode or a separate polarizing electrode located in proximity to the flame. However, this solution has not proved satisfactory for high temperature operation because microscopic particles of the quartz tend to break off during operation of the detector and enter the flame. The release of microscopic quartz particles causes sharp noise spikes, the magnitude and frequency of which are a function of a number of variables including the fuel gas ratios. It is believed that the microscopic particles are formed when thin sections of the quartz near the orifice of the jet are heated by the flame and undergo a phase transition resulting in a higher bulk volume and high localized stresses. It has been observed that the noise problems associated with quartz flame tips increase as the temperature of the system is increased.
Accordingly, it is an object of this invention to provide a flame jet for use in a flame detector in a gas chromatography system which forms a polarizing electrode and which is highly physically and chemically inert.
Another object of this invention is to provide a flame jet for use in a flame ionization detector in a gas chromatography system which provides high performance at temperatures as high as 500.degree. C.